Acrylic prepolymer and process for encapsulation of photocells

ABSTRACT

Acrylic prepolymer comprising: 
     from 10 to 50% by weight of units derived from at least one alkyl acrylate, the alkyl group having from 4 to 12 carbon atoms, 
     from 30 to 60% by weight of units derived from at least one alkyl methacrylate, the alkyl group having from 1 to 5 carbon atoms, and 
     from 10 to 40% by weight of units derived from methyl acrylate. 
     The prepolymer is used to encapsulate photocells, connected to one another by conducting wires and positioned on a support plate, by casting a resin into the space located between the support plate and a second protective plate, the resin being obtained by mixing 100 parts of the acrylic prepolymer, from 0.1 to 4 parts of a vanadium arenesulphonate and from 0.5 to 4 parts of a free-radical initiator, at a temperature between 10° and 70° C. and for a sufficient time to solidify the polymeric resin at the temperature selected.

BACKGROUND OF THE INVENTION

The present invention relates to an acrylic prepolymer and its use forencapsulating photocells.

A problem encountered in the manufacture of panels for utilizing solarenergy, referred to as solar panels, concerns the encapsulation of thephotocells supplying the panels. A material that is transparent,flexible, and very resistant to aging under the action of heat and/ormoisture is needed for this purpose.

SUMMARY OF THE INVENTION

The present invention overcomes this problem with an acrylic prepolymerthat has proved effective for the encapsulation of photocells, asdescribed in detail below.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve the foregoing objects and in accordance with the purpose ofthe invention, as embodied and broadly described herein, the acrylicprepolymer of the invention comprises:

from 10 to 50% by weight of units derived from at least one alkylacrylate, the alkyl group having from 4 to 12 carbon atoms,

from 30 to 60% by weight of units derived from at least one alkylmethacrylate, the alkyl group having from 1 to 5 carbon atoms, and

from 10 to 40% by weight of units derived from methyl acrylate.

Further to achieve the foregoing objects and in accordance with thepurpose of the invention, as embodied and broadly described herein, theinvention comprises a process for the manufacture of the above-describedacrylic prepolymer, comprising copolymerizing (a) from 10 to 50% byweight of at least one alkyl acrylate, the alkyl group having from 4 to12 carbon atoms, (b) from 30 to 60% by weight of at least one alkylmethacrylate, the alkyl group having from 1 to 5 carbon atoms, and (c)from 10 to 40% by weight of methyl acrylate. Copolymerization is carriedout in the presence of a free-radical initiator and a chain transferagent, at a temperature between 60° and 85° C., until a viscositybetween 1.5 and 10 poises is obtained.

Further to achieve the foregoing objects and in accordance with thepurpose of the invention, as embodied and broadly described herein, theprocess of the invention for encapsulating photocells comprisespositioning photocells connected to one another by conducting wires intoa space between a support plate and a second protective plate, andcasting a resin into the space. The resin is obtained by mixing 100parts of the above-described acrylic prepolymer containing 10 to 50% byweight of units derived from at least one alkyl acrylate, 30 to 60% byweight of units derived from at least one alkyl methacrylate, and 10 to40% by weight of units derived from methyl acrylate, with 0.1 to 4 partsof a vanadium arenesulphonate and from 0.5 to 4 parts of a free-radicalinitiator at a temperature between 10° and 70° C. and for a sufficienttime for the polymeric resin to solidify.

Still further to achieve the foregoing objects and in accordance withthe purpose of the invention, as embodied and broadly described herein,the invention comprises encapsulated photocells made by theabove-described casting process using the acrylic prepolymer of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferredembodiments of the invention.

The prepolymer according to the invention is a liquid that preferablyhas a density, at 20° C., of between 0.960 and 0.975 and a refractiveindex of between 1.43 and 1.44. It can also contain from 20 to 200 ppm(parts per million) of a polymerization inhibitor.

The process for the manufacture of the prepolymer according to theinvention comprises coplymerizing (a) from 10 to 50% by weight of atleast one alkyl acrylate, the alkyl group having from 4 to 12 carbonatoms, (b) from 30 to 60% by weight of at least one alkyl methacrylate,the alkyl group having from 1 to 5 carbon atoms, and (c) from 10 to 40%by weight of methyl acrylate, in the presence of a free-radicalinitiator and a chain transfer agent, at a temperature between 60° and85° C., until a viscosity of between 1.5 and 10 poises is obtained. Anexample of a chain transfer agent that can be used is a mercaptan, suchas laurylmercaptan, in an amount of 0.1 to 1% by weight, relative to allthe comonomers. Examples of free-radical initiators that can be used arean azo compound or a peroxide, such as lauryl peroxide, in an amount of0.2 to 2% by weight, relative to all the comonomers. If appropriate, themanufacture of the prepolymer according to the invention can be carriedout in the presence of a crosslinking agent--such as, for example,triethylene glycol diacrylate or methylacrylate, ethylene glycoldimethacrylate or butanediol diacrylate--and/or of at least one fourthcomonomer selected from the group consisting of acrylic acid,methacrylic acid, and ethylene glycol monomethacrylate.

A valuable application of the prepolymer according to the invention isfor encapsulating photocells in a flexible and transparent resinobtained by mixing:

100 parts of the prepolymer described above,

from 0.1 to 4 parts of a vanadium arenesulphonate, and

from 0.5 to 4 parts of a free-radical initiator,

at a temperature between 10° and 70° C. and for a sufficient time toobtain solidification of the polymeric resin at the temperature used. Amixing time between 1 and 24 hours is preferred.

The resin thus obtained preferably has a density, at 20° C., of between1.10 and 1.12, a refractive index (determined according to ISO StandardSpecification R 489) of between 1.47 and 1.48, and a Shore A hardness(determined according to ISO Standard Specification R 868) of between 20and 35. It is characterized by a remarkable resistance to atmosphericagents and in particular to aging under the action of heat and/ormoisture.

The vanadium arenesulphonate used in admixture with the prepolymer canbe dispersed beforehand in a diluent such as, for example, methylphthalate. The free-radical initiator can be selected from peroxides andperesters that can be decomposed at the selected temperature, such as,for example, methyl ethyl ketone peroxide, cyclohexanone peroxide,acetylacetone peroxide, tert-butyl hydroperoxide, butyl isopropylperoxycarbonate, tert-butyl peroctoate, and the like. The time requiredto obtain solidification of the polymer generally varies from 1 to 24hours, depending on the temperature in question.

In this preferred use of the prepolymer according to the invention, thephotocells, connected to one another by conducting wires, are generallypositioned first on a support plate. The mixture described above is thencast into the space located between the support plate and a secondprotective plate, which space can have a size on the order of 1 to 10mm, and the whole device is kept in a chamber at the desired temperaturefor the required time. The plates delimiting the space into which themixture is cast can be made of various materials such as glass, metal,or a synthetic resin such as polymethyl methacrylate. This results in aperfectly secure encapsulation of the photocells supplying the panelsintended for utilizing solar energy.

The examples which follow illustrate embodiments of the presentinvention without implying a limitation.

Example 1--Manufacture of the Acrylic Prepolymer

A mixture of monomers, consisting of 31% by weight of 2-ethylhexylacrylate, 47% by weight of ethylmethacrylate, and 22% by weight ofmethyl acrylate, was prepolymerized in the presence of 0.4% oflaurylmercaptan and 0.2% of lauryl peroxide. This reaction was carriedout at a temperature of 75° C. for a period of 90 minutes.

The prepolymer obtained was stabilized with the aid of 40 ppm of aninhibitor marketed under the name Topanol A.

The prepolymer obtained was characterized by the following physicalproperties:

Refractive index (ISO Standard Specification R 489): 1.435

Density at 20° C. (ISO Standard Specification 11-83): 0.968

Viscosity: 1.7 poises.

Example 2--Use of the Prepolymer for Encapsulation

Photocells were positioned on a support plate, and a 3 mm thick layer ofa mixture consisting of:

100 parts by weight of the prepolymer obtained according to Example 1,

2 parts by weight of a mixture of cyclohexanone peroxide and tert-butylperoctoate, in solution in methyl phthalate, and

2 parts by weight of vanadyl toluenesulphonate in solution in methylphthalate,

was then cast in the space located between the support plate and asecond protective plate.

The mixture was kept in this space at a temperature of 20° C. for 24hours. When this time had elapsed, the photocells were encapsulated in asolid polymer, the properties of which are described below.

Example 3--Properties of the Polymer

Immediate sampling of the polymer obtained by casting according toExample 2 made it possible to determine the following physicalproperties:

Refractive index

(ISO Standard Specification R 489): 1.475

Density at 20° C.

(ISO Standard Specification 11-83): 1.11

Shore A hardness

(ISO Standard Specification ISO R 868): 28

Flexibility: 148 seconds.

The flexibility was measured by compression on a tensile tester, theindicated value being the time in seconds taken to compress thetest-piece up to a load of 40 kg at a rate of 1.5 mm/minute.

Furthermore, the polymer was subjected to an aging test in anair-conditioned chamber filled with hot air (50° C.) having a moisturecontent of 50%. The light transmission, measured after a residence timeof 20 days in this chamber, was 94%. The polymer was also subjected toan aging test in water, by immersing it in a tank of hot water (50° C.).The flexibility, measured after a residence time of 20 days in thistank, was 148 seconds. It can therefore be noted that the polymerpossesses good resistance to aging under the action of agents such asheat and moisture.

It will be apparent to those skilled in the art that variousmodifications and variations could be made in the invention withoutdeparting from the scope or spirit of the invention.

What is claimed is:
 1. An acrylic prepolymer comprising:from 10 to 50%by weight of units derived from at least one alkyl acrylate, the alkylgroup having from 4 to 12 carbon atoms, from 30 to 60% by weight ofunits derived from at least one alkyl methacrylate, the alkyl grouphaving from 1 to 5 carbon atoms, and from 10 to 40% by weight of unitsderived from methyl acrylate.
 2. A prepolymer according to claim 1,possessing a density at 20° C. between 0.960 and 0.975.
 3. A prepolymeraccording to claim 1 or 2, possessing a refractive index between 1.43and 1.44.
 4. A prepolymer according to claim 1, further comprising from20 to 200 ppm of a polymerization inhibitor.
 5. A prepolymer accordingto claim 1 or 4, further comprising units derived from a fourthcomonomer selected from the group consisting of acrylic acid,methyacrylic acid, and ethylene glycol monomethacrylate.
 6. A processfor the manufacture of the acrylic prepolymer according to claim 1,comprising copolymerizing (a) from 10 to 50% by weight of at least onealkyl acrylate, the alkyl group having from 4 to 12 carbon atoms, (b)from 30 to 60% by weight of at least one alkyl methacrylate, the alkylgroup having from 1 to 5 carbon atoms, and (c) from 10 to 40% by weightof methyl acrylate in the presence of a free-radical initiator and achain transfer agent, at a temperature between 60° and 85° C., until aviscosity of between 1.5 and 10 poises is obtained.